1. Field of the Invention
The present invention relates to an automatic focusing camera and more particularly, to an automatic focusing camera with an automatic focusing function for automatically matching the focus in response to a video signal obtained from an image sensor, such as an electronic still camera.
2. Description of the Background Art
Conventionally, in an automatic focusing camera having an automatic focusing function such as a video camera and an electronic still camera, a focus controlling approach utilizing a video signal itself obtained from an image sensor for evaluating the state where the focus is controlled has been employed. According to such an approach, many good characteristics can be obtained. For example, there exists essentially no parallax. In addition, even if the depth of field is small and an object is located in the distance, the focus can be exactly matched. Furthermore, according to this approach, a specific sensor for automatic focusing need not be separately provided, so that the apparatus is very simple as a mechanism.
As an example of such a focus control method utilizing a video signal, a so-called hill-climbing servo system has been conventionally known. This hill-climbing servo system is described in, for example, U.S. Pat. No. 4,638,364 and U.S. Pat. No. 4,614,975, Japanese Patent Laying-Open No. 58-58505 and No. 60-103776. Briefly stated, a high frequency component of a video signal obtained from an image sensor is detected every one field as a focus evaluating value, and a focusing lens is moved back and forth in the direction of the optical axis so that the focus evaluating value always takes the maximal value.
FIG. 1 is a schematic block diagram showing an automatic focusing apparatus in an automatic focusing camera utilizing such a hill-climbing servo system, and FIG. 2 is a block diagram showing the details of a focus evaluating value generating circuit shown in FIG. 1. Such a conventional automatic focusing apparatus is disclosed in, for example, Japanese Patent Laying-Open No. 63-125910.
In FIG. 1, an automatic focusing camera comprises a focusing ring 2 for moving a focusing lens 1 back and forth in the direction of the optical axis, a focusing motor 3 for driving the focusing ring 2, a solid state image sensing device 45 such as a CCD (Charge Coupled Device), an image sensing circuit 4, and a timing generator 44. The focusing lens 1 may be moved using a piezoelectric device instead of a motor. In addition, the solid state image sensing device 45 such as a CCD itself may be moved back and forth instead of the focusing lens so as to change the relative position of the focusing lens 1 relative to the solid state image sensing device 45.
After an image formed on the surface of the CCD 45 by the focusing lens 1 is photoelectric-converted to an electric signal by the CCD 45, the signal is converted to a video signal by the image sensing circuit 4 and input to the focus evaluating value generating circuit 5. With reference to FIG. 2 indicating the details of the focus evaluating value generating circuit 5, a luminance signal component in a video signal output from the image sensing circuit 4 is applied to a synchronizing separator circuit 5a and a gate circuit 5c. The synchronizing separator circuit 5a separates a vertical synchronizing signal VD and a horizontal synchronizing signal HD from the input luminance signal and applies the separated signals VD and HD to a gate control circuit 5b. The gate control circuit 5b sets a rectangular sampling area FA as a focusing area in the central portion of a picture SX as indicated in FIG. 3, in response to the input vertical synchronizing signal VD and horizontal synchronizing signal HD and an output pulse from the timing generator 44 (FIG. 1) driving the CCD 45. The gate control circuit 5b applies a signal for opening or closing a gate for every field to the gate circuit 5c so that the luminance signal is allowed to pass only within the range of the sampling area.
Only the luminance signal corresponding to the range of the sampling area is applied to a high-pass filter 5d for every field by the gate circuit 5c. The high frequency component of the video signal separated by this high-pass filter 5d is amplitude-detected by a detector 5e, the detected output being applied to an A/D converter circuit 5f. The high frequency component converted into a digital value in a prescribed sampling cycle by the A/D converter circuit 5f (A/D converted data) is sequentially applied to an integrator circuit 5g.
The integrator circuit 5g is practically a so-called digital integrator composed of an adder (not shown) to add the input A/D converted data and the latch data of the succeeding latch circuit, and the latch circuit (not shown) to latch the output of the adder and to be reset for every field, and the data of the latch circuit immediately before it is reset, i.e. the sum of A/D converted data for the period of one field is output as a focus evaluating value. That is, the focus evaluating value generating circuit 5 extracts a luminance signal in the focus area in time dividing manner, then digitally integrates the high frequency component thereof over 1 field period and outputs the resulting integrated value as the focus evaluating value of the present field.
Immediately after the automatic focus operation is initiated, the focus evaluating value for the first one field output from the focus evaluating value generating circuit 5 is first applied to a maximum value memory 6 and an initial value memory 7 and held therein. Subsequently, a focusing motor control circuit 10 makes the focusing motor 3 rotate in the prescribed direction. Then a comparator 9 compares the initial focus evaluating value held in the initial value memory 7 and the focus evaluating value output from the focus evaluating value generating circuit 5 to generate a comparison signal, and the focusing motor control circuit 10 responds to this signal and carries out the initialization of the rotating direction of the focusing motor 3.
That is, the focusing motor control circuit 10 continues to make the focusing motor 3 rotate in the above mentioned prescribed direction, until the comparator 9 generates a comparison output indicating "large" or "small". If the comparator 9 outputs a compared output indicating that the later obtained focus evaluating value is larger exceeding the prescribed fluctuation range compared to the initial focus evaluating value held in the initial value memory 7, the focusing motor control circuit 10 maintains the above mentioned prescribed rotating direction as it is. Meanwhile, if an comparison output is obtained indicating that the later obtained focus evaluating value is smaller exceeding the prescribed fluctuation range compared to the initial focus evaluating value, the focusing motor control circuit 10 reverses the rotating direction of the focus motor 3.
Thus, the initialization of the rotating direction of the focussing motor 3 is completed, the focusing motor control circuit 10 monitors the output of a comparator 8 from this time on.
Meanwhile, the comparator 8 compares the maximum focus evaluating value so far held in the maximum memory 6 and the current focus evaluating value output from the focus evaluating value generating circuit 5 and outputs two kinds of comparison signals (P1, P2), i.e. in the case in which the current focus evaluating value is larger than the focus evaluating value held in the maximum memory 6 (the first mode), or the case in which the same is reduced exceeding the prescribed threshold value .DELTA.y (the second mode). Now, in case the current focus evaluating value is larger than the content of the maximum value memory 6, the content of the maximum value memory 6 is updated in response to the output of the comparator 8 (P1) whereby the maximum value of the focus evaluating value so far is always held in the maximum value memory 6.
In response to the position of the focusing ring 2 supporting the focusing lens 1, a focus ring position signal is generated from the focusing ring 2 and this focusing ring position signal is applied to a focusing ring position memory 13. This focusing ring position memory 13 is updated in response to the output of the comparator 8 so as to always hold the focusing ring position signal at the time when the focus evaluating value reaches the maximum. The focusing ring position signal is generally output from a potentiometer (not shown) provided to detect the focus ring position. However, it is also possible to utilize a stepping motor as the focusing motor 3 and to detect the amount of rotation of this motor in the near point direction and the infinity point direction as positive and negative stepping amount respectively thereby indicating the focusing ring position or focusing motor position by this stepping amount.
The focusing motor control circuit 10 monitors the output of the comparator 8 while rotating the focusing motor 3 in the direction initialized in response to the output of the comparator 9 as described above. With reference to FIG. 4 indicating the relation between the positions of the lens and the focus evaluating value, when the comparison output (P2) of the second mode indicating the current focus evaluating value is reduced exceeding the above mentioned threshold value .DELTA.y compared to the maximum focus evaluating value, the focusing motor control circuit 10 reverses the rotating direction of the focusing motor 3. The focusing motor is first reversed when the focus evaluating value is reduced exceeding the prescribed threshold value for the purpose of preventing the erroneous operation caused by the noise of the focus evaluating value.
After the focusing motor 3 is reversed, comparison is conducted in a comparator 14 between the content of the focusing ring position memory 13 which corresponds to the maximum value of the focus evaluating values and the current focusing ring position signal generated from the focusing ring 2. When a matching is obtained, i.e. when the focusing ring 2 is returned to the position in which the focus evaluating value reaches its maximum, the focusing motor control circuit 10 stops the rotation of the focusing motor 3. At the same time, the focusing motor control circuit 10 outputs a lens stop signal LS. A series of automatic focusing operation is thus completed.
FIG. 5 is a flow chart showing a case in which the above mentioned series of automatic focusing operation is carried out in a software manner utilizing a microcomputer. Since the detail is just as mentioned above, therefore the description is not repeated here.
A video camera is a typical example of a camera which needs to be always supplied with a video signal successively as mentioned above, and particularly in a video camera employing the automatic focusing function of a hill-climbing servo system, the automatic focusing operation is carried out utilizing the above mentioned successively supplied video signal as a material for judgement. A description will be given in the following on the operating method of the CCD 45 by the timing generator 44. FIG. 6 is a schematic diagram indicating a general structure of the CCD 45. As CCDs, the one employing a known interline system and the other one employing a known frame transfer system are representative, and the description in the following is related to a CCD employing the interline system which is often used for video cameras.
In FIG. 6, the CCD 45 is provided with an array composed of a plurality of photosensors Ph arranged in the two-dimension of the vertical direction (column direction) and the horizontal direction (the row direction). Here, in the CCD of FIG. 6, for the purpose of description, the array is described having a structure that each row is formed of 8 photosensors Ph and each column is formed of 10 photosensors Ph, but in a CCD used for an actual video camera, a large number of photosensors, for example 510 in each row, and 480 in each column, are formed.
Corresponding to each photosensor Ph, a register VR is formed and the photosensor Ph and the register VR are coupled by a sensor gate. The output electric charges caused by the photoelectric conversion operation by each photosensor Ph are extracted through the sensor gate to the corresponding register VR. The registers VR in each column are coupled in the vertical direction to form a vertical transfer portion, which sequentially transfers the electric charges extracted from respective photosensors Ph in the vertical direction. And then the charges transferred to the register located in the lowest end of the vertical transfer portion in each column are extracted to a register HR which corresponds to each column. These registers HR are coupled in the horizontal direction to form a horizontal transfer portion, which supplies the output charges in respective rows to an output portion 43.
The timing generator 44 generates sensor gate pulses SG, vertical transfer pulses VG and horizontal transfer pulses HG, and these are used for the extraction of charges from the photosensors Ph, the transfer control of charges in the vertical transfer portion and the transfer control of charges in the horizontal transfer portion, respectively. FIG. 7 is a waveform diagram showing the charge transfer operation of the CCD using these various signals.
First, a sensor gate pulse SG is generated from the timing generator 44 in synchronization with a vertical synchronizing signal VD, i.e. once every one field (1/60 seconds), and the same is supplied to all the sensor gates constituting the array of FIG. 6 simultaneously and commonly whereby the signal charges of the photosensors are simultaneously transferred to the corresponding registers VR respectively.
Next, when a vertical transfer pulse VG is generated from the timing generator 44 for every one horizontal scanning period (1H), the signal charges held in respective registers VR constituting the vertical transfer portion are transferred by one row for every 1H in the vertical direction (downward direction), and transferred from the lower end of each vertical transfer portion to the register HR of the horizontal transfer portion on a row basis. Here, the vertical transfer pulses VG includes 10 pulses for every one field period, and this is because each column includes 10 photosensors Ph in the CCD of FIG. 6. That is, all the signal charges read out simultaneously from the photosensors PH to the registers VR by sensor gate pulses are to be transferred to the horizontal transfer portion over the period of one field.
Meanwhile, the charges of each row transferred to the horizontal transfer portion from the lower ends of the vertical transfer portions are sequentially transferred to the output portion 43 for every 1H, in response to the horizontal transfer pulse HG from the timing generator 44.
In the above described manner, in response to respective pulses output from the timing generator 44, signal charges for every row are sequentially extracted from the CCD 45 to the output portion 43, and after being amplified at the output portion 43, the charges are applied to the signal processing portion (not shown) in the image sensing circuit (FIG. 1) in the successive stage. The signal processing portion carries out the prescribed signal processing on these signal charges and outputs a video signal.
Here, the above mentioned operating method of the CCD is presented schematically for the purpose of easily describing the basic operating principle, and therefore in practice the vertical transfer portion is structured such that signals for two rows are added to each other and then transferred to the horizontal transfer portion, in order to realize interlace operation.
As mentioned above, the general operating method of a CCD requires one field period for reading out the signal charges from all of the photosensors constituting the CCD, and therefore a focus evaluating value to be used for the automatic focusing operation is available only once for every one field.
On the other hand, in order to carry out the above mentioned automatic focusing operation in accordance with a hill-climbing servo system at high speed, it is inevitable to increase the rotating speed of the focusing motor 3 (FIG. 1). Now, in case the rotating speed of the focusing motor 3 is simply increased, the amount of movement of lens during one field period is increased accordingly, but since the calculation of a focus evaluating value is always carried out only once for every one field period, the frequency of detection of the focus evaluating value with respect to the amount of movement of the lens decreases. Therefore, the automatic focusing control becomes rough thereby decreasing the accuracy of focusing remarkably. Consequently, there emerges a certain limit to the high speed driving of the focus motor.
Described further in detail, for example, in case a lens of F2. 8 is used at a focal distance of 30 mm for a solid state image sensing device of 2/3 inches, at least about 2 seconds are necessary to shift the focusing ring over the entire range possible to be focused by the lens, i.e. the range between the position close to the camera and the position at infinity, in order to maintain the high accuracy in the automatic focusing operation in accordance with the above mentioned hill-climbing servo system. This is because the focus evaluating value needs to be detected at least 120 times usually in order to carry out the focusing operation with the necessary accuracy in such a range, and also as described in the foregoing, one field period i.e. 1/60 seconds is necessary for one detection.
Usually in a video camera, as the image sensing is successively performed, it is not very much troublesome in practice to spend a little bit of time for focusing. However, in case such an automatic focusing function is applied to an electronic still camera in which image sensing the state is not continued, the fact that at least about 2 seconds are necessary for focusing becomes a big problem in practice. That is, generally in a still camera if the time between pressing the releasing button (the shutter button) and the completion of image sensing is not as short as about 0.5 seconds, the user recognizes the releasing time lag and the camera will be very difficult to handle for him. Here, if it is assumed that, of the above mentioned 0.5 seconds, about 0.4 seconds is needed for a focusing operation, it is necessary to restrain the time to 1/5 of two seconds which is usually necessary for focusing. However, as described in the foregoing, for the frequency of the calculation of focus evaluating values is constant, such significant reduction of focusing time is difficult to realize in accordance with the conventional technology. Therefore, considering the focusing time, the conventional automatic focusing system in which the focus evaluating value is detected only once every one field period can not be employed for use in an electronic still camera.